The present invention relates to an apparatus for preventing the locking of a wheel on a vehicle such as a motorcycle, automobile or the like, and more particularly to such a wheel locking prevention apparatus including a brake-fluid pressure transmission system bypassed by a reservoir mechanism for releasing the brake fluid pressure and a pump mechanism for returning the brake fluid from the reservoir mechanism to the brake-fluid pressure transmission system.
There are known antiskid apparatus having a pneumatically operated large-size pressure-reducing device. One type of proposed antiskid apparatus has a directional control valve assembly for releasing the pressurized fluid from a brake-fluid pressure transmission system into a reservoir mechanism when the brake fluid pressure is to be reduced, and for enabling a pump mechanism to pump the fluid from the reservoir mechanism back to the brake-fluid pressure transmission system when the brake fluid pressure is to be increased again. Thus, the antiskid apparatus of this construction serves as a wheel lock prevention apparatus. This type of antiskid apparatus is suitable for use in a design requiring a small-size structure.
The wheel lock prevention apparatus with such a directional control valve assembly has the following construction:
The brake-fluid pressure transmission system (hereinafter referred to as a "main passage") for interconnecting a master cylinder serving as a source for generating a fluid pressure and brake units includes a first normally-open directional control valve (hereinafter referred to as a "first valve") for closing the main passage when the brake fluid pressure is to be reduced. The main passage is bypassed by another passage (hereinafter referred to as a "bypass passage"). The bypass passage and the main passage are interconnected by a second normally-closed directional control valve (hereinafter referred to as a "second valve") which opens when the brake fluid pressure is to be dropped. To the bypass passage, there are connected a reservoir mechanism for reserving the brake fluid at a decreasing pressure while increasing the volume of the reservoir space in response to the pressure of the brake fluid, and a pump mechanism for pumping the brake fluid from the reservoir mechanism into the main passage. The first and second valves are actuated by an electronic control circuit for detecting the locking of a wheel when the automobile is braked.
The pressure of the braking fluid will be controlled by the foregoing wheel lock prevention apparatus as follows: When the wheel braking power exerted at the time the vechile is braked is too large due to a reduced coefficient of friction of the road surface, the speed of rotation of the wheels is abruptly reduced and the wheel is locked, resulting in the danger of losing the steerability of the vehicle. Under such conditions, the wheel lock prevention apparatus controls the pressure of the brake fluid to be lowered to an optimum level. If the brake fluid pressure were reduced excessively, then the wheels would not be locked, but the vehicle would run for an increased distance before it would be stopped completely. Therefore, it is preferable for the wheel lock prevention apparatus to lower the rotational speed of the wheels while maintaining a suitable rate of slippage between the wheels and the road surface. The wheel lock prevention apparatus operates while the vehicle is being braked through depression of the brake pedal by the driver. Since the wheel speed tends to drop in quite a short period of time when the brake pedal is depressed, the wheel lock prevention apparatus is required to be highly responsive.